This invention relates to a transversal type equalizer apparatus utilized for communication systems or lines, and more particularly a transversal type equalizer apparatus provided with a control circuit for setting an optimum tap coefficient of an equalizer.
Generally, in communication systems, such transmission distortions as amplitude distortion, delay distortion or the like are caused owing to imperfectness of devices utilized for communication. For this reason, in order to maintain a stable communication system, a certain type of distortion equalization is necessary.
Especially, in a satellite communication system, a high power amplifier is mounted on a satellite. However, since the type is of a non-linear amplifier, equalization of transmission distortion carried out in either the transmitting apparatus or receiving apparatus is insufficient to maintain a stable communication state, as well known in the art.
For this reason, in a communication system including a non-linear amplifier, independent equalizers are provided for the transmitting and receiving apparatus to perform independent transmission path equalization before and after the non-linear amplifier. One example of the distortion equalizer is a transversal type equalizer.
To have better understanding, one example of the construction of a transversal type equalizer, to which the present invention is applicable, will be described with reference to FIG. 1. It comprises a signal input terminal 1, 2N delay circuits 2 each having a predetermined delay time T.sub.0, (2N+1) complex multipliers 3, a signal combiner 4 in the form of an adder circuit and an output terminal 5. Complex signals C.sub.-N, C.sub.-N+1 . . . C.sub.N-1, and C.sub.N representing tap coefficients or tap gain control signals, where N represents a positive interger, are applied to the complex multipliers. The frequency characteristic of this equalizer is shown by the following equation: ##EQU1##
With this equalizer, it is possible to vary the frequency characteristic without interrupting the signal by varying the tap coefficients C.sub.n and hence, this equalizer is effective for use as an equalizer in a communication circuit. As a criterion for evaluating the equalizing effect for the circuit distortion, flatness of the frequency characteristics (amplitude, delay) is generally used. In a circuit in which it is difficult to correctly grasp the equalization characteristic owing to the limit of a measuring system, a method is used wherein the circuit distortion is equalized so as to make optimum the quality of the output signal of a receiver. In such a circuit, the above-described equalizer is effective because the circuit can be equalized without interrupting signals.
One of the serious problems in the operation of the transversed type equalizer is encountered in selecting respective tap coefficients C.sub.n so that a desired characteristic T(.omega.) of the equalizer can be obtained. Especially, where the equalizer is used as an amplitude or delay equalizer, there is no effective measure to know its characteristic when the circuit is used in the on-line mode. Even if there is a measure for knowing the characteristic, the combinations of respective tap coefficients C.sub.n are numerous with the result that the measure for equalization can not be automated for the equalizer on the transmission side. This requires a large adjusting time. To cope with this difficulty, another measure is conceivable in which respective tap coefficients are precalculated and stored. By the latter measure, however, an extremely large number of memory devices are necessary, thus increasing the cost of the system.
Taking a TDMA (time division multiple access) satellite communication system, for instance, the equalizing characteristic will be as follows:
______________________________________ Parabolic group delay 0.00 to (-0.025)ns/MHz.sup.2 Linear delay 0.00 to (.+-.0.25)ns/MHz Linear amplitude 0.00 to (.+-.0.05)dB/MHz Parabolic amplitude 0.00 to (.+-.0.002)dB/MHz.sup.2 ______________________________________
In this system, the transmission path bandwidth is about .+-.40 Mz so that it is necessary to sufficiently equalize in this range. However, the range of equalization in .+-.30 Mz, is 0 to (-22.5)ns for the parabolic group delay, .+-.7.5 ns for the linear group delay, +1.5 dB for the linear amplitude and .+-.1.8 dB for the parabolic amplitude. It is not always necessary to provide continous correction within this range, and it is known that even when the residual distortion after equalization is considered, the code error rate of the demodulated signal after the correction by means of the transversal type equalizer can be maintained in a sufficient expected-value range if the correction could be made by 0.1 dB for the amplitude characteristic, and every 0.5 ns or 1 ns for the group delay characteristic. In this case, for each tap, 46 words are necessary for the parabolic group delay, 31 words for the linear group delay, 31 words for the linear amplitude, and 37 words for the parabolic amplitude. Where all combinations of respective characteristics are to be equalized, at least 45.times.30.times.30.times.36=1458000 words are necessary, but the limit of the number of memory words now being used is 8K words (where a 64K ROM is used.)
For this reason, the aforementioned system or a so-called memory table reference system in which all of the previously calculated tap coefficients are stored in the memory device and they are read out when necessary is difficult to realize through currently available technique.
On the other hand, a further measure may be considered wherein a necessary number of optimum tap coefficients are calculated when desired, but such a measure is also difficult to realize from the standpoint of calculation time and memory size.